Recently, miniaturization and high quality of solid-state imaging elements constituting a solid-state imaging device have been developed, and the technique for manufacturing solid-state imaging devices has also been made finer. Accordingly, improving the sensitivity and reducing smear have been required.
An example of a solid-state imaging device of the prior art is hereby explained with reference to the drawings (FIGS. 11 to 14).
FIG. 11 is an enlarged plan view of a part of a light receiving region of a solid-state imaging device of the prior art. FIG. 12 is a cross sectional view taken along line X--X of FIG. 11, and FIG. 13 is a sectional view taken along line Y--Y of FIG. 11. FIG. 14 shows a potential distribution at a cross section taken along line A"-A-B-A' of FIG. 11 when charges are stored.
As is apparent from the above figures, the solid-state imaging device of the prior art comprises a semiconductor substrate 1, a photodiode 2, a vertical charge coupled device (CCD) 3, an isolation area 4 between CCDs, an isolation area 5 between photodiodes, a polysilicon electrode 6, an oxide film 7, a metal shielding film 8, an aperture 10 of the shielding film, and an insulating film 11. Moreover, each figure illustrates the motion of signal charges 12 generated by incident light 9.
Next, specific structure of the above mentioned solid-state imaging device is explained.
The semiconductor substrate 1 is a silicon mono-crystal. A p-type well is formed on a part forming the light receiving region by implantation of ions. The photodiode 2 is formed in the p-type well by implantation of n-type arsenic, phosphorus, or the like. Similarly, the vertical CCD 3 is formed in the p-type well by implantation of n-type arsenic, phosphorus, or the like.
The isolation area 4 between CCDs is formed between the photodiode 2 and the vertical CCD 3, which forms a potential barrier by implantation of p-type boron, etc. Similarly, the isolation area 5 between photodiodes is formed between neighboring photodiodes 2, which forms a potential barrier by implantation of p-type boron, etc.
The polysilicon electrode 6 is formed on the vertical CCD 3. By this polysilicon electrode 6, the potential of the vertical CCD 3 is controlled. The oxide film 7 is formed on the polysilicon electrode 6 in such a manner that it covers a surface of the polysilicon electrode 6. The metal shielding film 8 is formed on the oxide film 7 and shields the vertical CCD 3 from incident light 9 to prevent smearing. The aperture 10 of the shielding film for receiving incident light 9 is formed on the photodiode 2. The insulating film 11 is formed on the metal shielding film 8. This insulating film 11 protects the solid-state imaging device.
In the solid-state imaging device according to the prior art having the above mentioned structure, signal charges are generated through photoelectric conversion of the incident light 9 by the photodiode 2 formed on the semiconductor substrate 1. Then, these signal charges are read out to the vertical CCD 3.
However, in the solid-state imaging device according to the prior art, if the p-type concentration of the isolation area between photodiodes is compared with the p-type concentration of the isolation area between CCDs, then the dose of implanted ions to the isolation area between photodiodes is greater than that to the isolation area between CCDs. Consequently, when charges are stored, the potential barrier of the isolation area 5 between photodiodes is higher than the potential barrier of the isolation area between CCDs (see FIG. 14). Thus, since the potential barrier of the isolation area between CCDs (point B) is low, signal charges generated by incident light to the isolation area between photodiodes (point A) are not stored in the photodiode but directly enter (spill over into) the vertical CCD. As a result, the problem of smearing occurs due to these signal charges spilling over into the vertical CCDs.
In order to avoid the above problems, a shielding film is usually formed on the isolation area between photodiodes so as to shield the incident light. However, if a shielding film is formed on the isolation area between photodiodes in consideration of a process margin for preventing dislocation when a mask of the aperture for the photodiode is aligned, or a coverage of the step difference of a base with the shielding film, then the width of the aperture of the shielding film is considerably limited and the effective area of the photodiode is reduced, thus causing a problem that hindered improvement of sensitivity.
Moreover, even if the shielding film is formed on the isolation area between photodiodes, there arises a problem that signal charges generated by a part of the incident light passing through the aperture are directly taken into the vertical CCD due to drift, and smear occurs. One way of avoiding this problem is to reduce the area of the aperture of the shielding film. Reducing the area of the aperture of the shielding film deteriorates the sensitivity of the solid-state imaging device.